Method for making a phosphor screen of a cathode-ray tube

ABSTRACT

A layer through which light can pass subject to some attenuation is provided on that surface of a shadow mask which opposes to a phosphor screen within a cathode-ray tube, a photosensitive layer is provided on the first-mentioned layer, light-permeating apertures smaller than the light-permeating openings of the shadow mask are formed in the photosensitive layer, and phosphor dots are formed using the assembly thus obtained. Thereby reduction of the manufacturing cost of CRT&#39;&#39;s and high improvements in the performance of the same are accomplished.

United States Patent [191 Takemoto et al.

[ Nov. 27, 1973 METHOD FOR MAKING A PHOSPHOR SCREEN OF A CATHODE-RAY TUBE [75] Inventors: Takeo Takemoto; Yoshiiumi Tomita,

both of Mobara, Japan [30] Foreign Application Priority Data Dec. 4, 1970 Japan 45/106845 [52] US. Cl 96/36.1, 96/44, 96/45 [51] Int. Cl. G03c 5/00 [58] Field of Search 96/36.l, 36, 44, 96/45 [56] References Cited UNITED STATES PATENTS 3,653,901 4/1972 Etter 96/36.l

12/1962 Schwartz 96/36.1 11/1960 Amdursky et a1. 96/36.l

Primary Examiner-Norman G. Torchin Assistant Examiner-Edward C. Kimlin Atz0rneyPaul M. Craig et a1.

[5 7 ABSTRACT A layer through which light can pass subject to some attenuation is provided on that surface of a shadow mask which opposes to a phosphor screen within a cathode-ray tube, a photosensitive layer is provided on the first-mentioned layer, light-permeating apertures smaller than the light-permeating openings of the shadow mask are formed in the photosensitive layer, and phosphor dots are formed using the assembly thus obtained. Thereby reduction of the manufacturing cost of CRTs and high improvements in the performance of the same are accomplished.

6 Claims, 2 Drawing Figures METHOD FOR MAKING A PHOSPHOR SCREEN OF A CATI'IODE-RAY TUBE This invention relates to a method for making a phosphor screen of a cathode-ray tube.

A recent shadow-mask type color cathode-ray tube has been improved in brightness and contrast with such a construction that phosphor dots smaller in diameter than openings of a shadow mask are formed on the inner surface of a faceplate, and that the interstice among the phosphor dots is filled up with a colored material which absorbs external light.

Generally employed in order to form phosphor dots in this case is, for example, a method in which a film of the colored material absorbing external light is applied, apertures smaller in diameter than the openings of the shadow mask are formed at positions for forming the phosphor dots thereat, and 'the phosphor dots are formed in conformity with the pattern of the smaller apertures. More specifically, a photoresist film on the inner surface of the faceplate is exposed to light through a certain pattern ofmask holes to form photoresist dots smaller in diameter than the mask holes. Thereafter, the colored material is applied on that part of the inner surface of the faceplate which is not covered, and on the photoresist dots. The photoresist dots are then removed to obtain'a predetermined diameter of phosphor dots. Further, according to the so-called exposure process from the outside of the faceplate, phosphor dots are first applied and formed on the inner surface of the faceplate, there-after the colored material is applied onto the entire inner surface of the faceplate including the phosphor dots, and exposure is carried out under this state'from the outer surface of the faceplate. A variety of such methods have been heretofore proposed. 7

As the shadow mask employed when the phosphor screen is formed by the above-mentioned methods,

there is generally used such as stated .below. The shadow mask has a smaller diameter of openings than a predetermined one, and therefore requires post etching, namely, after completion of a predetermined exposure step, etching is carried out with such an etchant as a solution of ferric chloride to make the smaller diameter the predetermined one.

Since, however, such shadow mask to be post-etched, having been already used for the formation of the phosphor dots, is provided with a mask frame, springs, etc. and is of a curved shape'so as to be actually assembled into a cathode-ray tube. For this reason, it is difficult to perform uniform etching over the entire surface of the shadow mask. In addition, in some etching methods, it is required to take corrosion inhibiting measures for the mask frame, the springs, etc. Moreover, an extremely careful consideration should be given to maintenance of the precision of the diameter of the openings of the shadow mask.

Further, with the method requiring the post-etching, the faceplate of each cathode-ray tube has the phosphor dots formed in conformity with the corresponding shadow mask. Therefore, if a defective unit is caused at, for example, the post-etching, a faceplate whose phosphor dots are formed by the defective shadow mask becomes also defective. The economical advantages of the method are accordingly questionable.

In order to solve the problems as mentioned above, this invention is characterized in that a shadow mask is covered .with a photosensitive layer and a lightpermeating and -attenuating layer to form a shadow mask assembly, and that a phosphor layer on the inner surface of a faceplate is exposed to light through the shadow mask assembly which, as will be hereinafter stated, has a desired phosphor-dot pattern of effective light-passing apertures of a diameter temporarily made smaller than that of openings of the shadow mask.

According to this invention, there is provided a method for making a phosphor screen of a cathode-ray tube comprising the steps of: providing a layer, through which light can pass subject to some attenuation, on

that surface of a shadow mask which opposes to said phosphor screen within said cathode-ray tube, said shadow mask being provided with a pattern of lightpermeating openings; providing a photosenstive layer on the light-permeating and -attenuating layer; exposshadow mask assembly; exposing a phosphor layer to light using said shadow mask assembly obtained; and forming a phosphor-dot pattern by a developing treatment. The method for making a phosphor screen of the present invention offers such advantages that the di mensional accuracy of applied and formed phosphor, dots is high and the production cost is lowered, eliminating various problems attendant upon the partial prevention of corrosion of the shadow mask, maintenance of the diameter of the openings of the shadow mask, reetching of the shadow mask after an exposure opera tion, and so forth.

This invention will now be described in detail with reference to the accompanying drawings. For illustrating an embodiment of the invention, in which FIG. 1 is a sectional view of a part of a shadow mask assembly before it is exposed to light, while FIG. 2 is a sectional view of a similar part of the shadow mask assembly shown in FIG. 1 but after it has been exposed to light.

First, a shadow mask 1 having a predetermined diameter of openings 10 is prepared. A photosensitive sheet 2 consisting of a photosensitive layer 20 and a lightpermeating and -attenuating layer 2b (hereinbelow termed the transparent attenuation sheet) is stuck through a transparent binding layer 3 to that surface 1a of the shadow mask I which opposes to a phosphor screen, so closely that the photosensitive sheet 2 may not become crumpled. The photosensitive layer 20 is closer to the phosphor screen than the transparent sheet 2b. The close adherence may also be directly made without the binding layer 3 as mentioned above, by the use of the method of, e. g.,- thermocompression bonding or vacuum adherence. The photosensitive layer 20 may be of a material, such as polyvinyl chloride, which has the property of causing the reaction of photopolymerization upon irradiating light thereon, being highly polymerized and losing tackiness. In addition, there may also be used a material which, as in polyvinyl carbazole, has the property of being changed in the charged state upon irradiation with light. Next, using a high-voltage mercury-arc lamp (not shown), the photosensitive sheet 2 is exposed to light through each opening 10 of the shadow mask 1 from the electron-gun side lb of the shadow mask 1 as is illustrated by an arrow in FIG. 1. The light having passed through the openings 10 of the shadow mask 1 at the exposure, is transmitted through the transparent attenuation sheet 2b and reaches the photosensitive layer 20. In the course of the transmission through the transparent attenuation sheet 2b, the light is attenuated by absorption and scattering. As a result, as shown at 4 in FIG. 2, each light-exposed portion of the photosensitive layer 2a which is located in correspondence with the opening 10 of the shadow mask 1, Le, the effective light-passing aperture of the shadow mask, becomes smaller in diameter than the opening 1c of the shadow mask 1.

Subsequently, particles of a light-impermeable material 5 such as carbon and graphite and having a particle size of 20 60 millimicrons or so are uniformly sprinkled over the photosensitive layer 2a. After the lightimpermeable material 5 is sprinkled, the powder on the light-exposed portions 4 is removed by a well-known method such as air blow and solvent immersion. Thus, the exposed portions 4 becomes light-permeable, a covering layer of the light-impermeable material 5 is formed at the part other than the exposed portions 4, and the effective light-passing aperture smaller than the openings of the shadow mask 1 are provided in the photosensitive sheet 21. Herein, the fixation of the layer of the light-impermeable material 5 may, if necessary, be made with a bonding agent, etc.

The provision of the photosensitive sheet will now be described in more detail. In case where the photosensitive layer 2a is a tacky one as made of the aforesaid polyvinyl chloride, the light-exposed portions 4 lose tackiness by the exposure, and hence, the light-impermeable material 5 does not adhere to the portions. For this reason, the light-permeating portions are formed. An example of the diameter of the effective light-passing apertures was attained by such method, as below. The diameter of the openings 10 of the shadow mask was 0.32mm, the thickness of the transparent attenuation sheet 2b was 8,1, and the thickness of the photosensitive layer 2a was 1;!" The exposure was carried out with a mercury-arc lamp of 400W located at a position distant by approximately 30cm from the shadow mask 1 1. Thereafter, carbon powder of, e.g., mg. was sprinkled, and the air blow was performed. As a result, the diameter of the light-exposed portions 4 became approximately 0.23mm.

In addition to the above method using the tacky photosensitive layer, there are a variety of methods. For example, according to a method, electric charges in the exposure portions 4 are caused to disappear by the exposure, so as to make the portions repel the lightimpermeable material, whereby the effective lightpassing apertures are formed. In another method, the photosensitive layer may contain a mixture consisting of a material such as the above-mentioned polyvinyl chloride and a light-impermeable material such as carbon, and the exposure portions 4 are deprived of tackiness by the exposure. In a further method, particles of the light-impermeable material are sprinkled on the photosensitive layer being tacky, both the particles and the layer are previously brought into the adhering state, and thereafter the light-impenneable material at the exposure portions 4 is removed by the exposure.

An exposure for forming a phosphor screen is carried out through the shadow mask assembly 11 having the photosensitive sheet 21 as described above, and the phosphor screen is made. Thus, a faceplate for cathode-ray tubes is provided which has phosphor dots smaller in diameter than the openings of the shadow mask 1. According to this method, the faceplate may be manufactured far more easily than in the prior-art post etching process previously stated. The shadow mask assembly 11 is restored to the desired shadow mask 1 by removing the photosensitive sheet 2] after completion of the exposure.

While, in the foregoing, description has been made of the embodiment for the shadow-mask type color cathode-ray tube, the invention is easily applicable to, for example, a Chromatron (trade name) type color cathode-ray tube besides.

What we claim is:

l. A method for making a phosphor screen of a cathode-ray tube comprising the steps of:

providing a transparent layer, through which light can pass subject to some attenuation, on that surface of a shadow mask which opposes the phosphor screen within said cathode-ray tube, said shadow mask being provided with a pattern of lightpermeating openings;

providing a photosensitive layer of a material capable of forming light-permeating apertures on the transparent layer;

exposing said phososensitive layer to light through said shadow mask and said transparent layer on said shadow mask, to form light-exposed portions smaller in diameter than said light-permeating openings of. said shadow mask in said photosensitive layer;

developing said light-exposed portions by applying light impermeable material to the photosensitive layer and by removing the light impermeable material from said light-exposed portions to form effective light-permeating apertures corresponding to said light-exposed portions and thereby to obtain a shadow mask assembly;

exposing a phosphor layer to light using said shadow mask assembly; and

forming a phosphor-dot pattern by a developing treatment of said light-exposed phosphor layer.

2. The method according to claim 1, wherein said photosensitive layer is a tacky photosensitive layer of polyvinyl chloride containing light-impermeable particles which looses tackiness by exposure to light whereby said light-impermeable particles may be removed therefrom during said development.

3. The method according to claim 1, wherein said photosensitive layer has a property of being changed in the charged state by exposure to light, and a lightimperrneable particle layer is provided on said photosensitive layer before exposing said photosensitive layer to light.

4. The method according to claim 1, wherein said photosensitive layer is a tacky photosensitive layer losing tackiness by exposure to light, and a lightimpermeable particle layer is provided on that part of said photosensitive layer at which the tackiness is not lost, after exposing said photosensitive layer to light.

5. The method according to claim 3, wherein the photosensitive layer is made of polyvinyl carbazole and the light-impermeable particle layer is formed of carbon particles.

6. The method according to claim 4, wherein said photosensitive layer is made of polyvinyl chloride and said light-impermeable particle layer is formed of carbon particles. 

2. The method according to claim 1, wherein said photosensitive layer is a tacky photosensitive layer of polyvinyl chloride containing light-impermeable particles which looses tackiness by exposure to light whereby said light-impermeable particles may be removed therefrom during said development.
 3. The method according to claim 1, wherein said photosensitive layer has a property of being changed in the charged state by exposure to light, and a light-impermeable particle layer is provided on said photosensitive layer before exposing said photosensitive layer to light.
 4. The method according to claim 1, wherein said photosensitive layer is a tacky photosensitive layer losing tackiness by exposure to light, and a light-impermeable particle layer is provided on that part of said photosensitive layer at which the tackiness is not lost, after exposing said photosensitive layer to light.
 5. The method according to claim 3, wherein the photosensitive layer is made of polyvinyl carbazole and the light-impermeable particle layer is formed of carbon particles.
 6. The method according to claim 4, wherein said photosensitive layer is made of polyvinyl chloride and said light-impermeable particle layer is formed of carbon particles. 